The present chargers for handsets and digital cameras mostly adopt a switch mode power supply. Such a charger has a PWM controller on the primary side of a transformer to control electric power according a feedback signal. The secondary side of the transformer has a dual processing amplifier circuit to provide a constant current and a voltage base circuit to provide a constant voltage, or a transistor to provide the constant current and a voltage base circuit to provide the constant voltage. However in order to save cost, in practice the feedback signal end of the PWM controller and the power supply end are coupled together. Refer to FIG. 1 for a block diagram of a conventional switch mode power supply 100. It includes a commutation filter device 102 to receive electric power from a city power and perform electric power rectification and filtering, a power switch 104 which is electrically connected to the commutation filter device 102, a transformer 106 which has a primary side electrically connected to the power switch 104, an output commutation filter device 108 which is electrically connected to a secondary side of the transformer 106, a PWM control device 110 to get a feedback signal and electric power from the output commutation filter device 108 through a coupling element 112 and output an electric power signal to the power switch 104. The PWM control device 110 and the coupling element 112 form a feedback circuit.
While the PWM controller gets the feedback signal and electric power from the same input end and can save cost, it does not provide voltage overshoot protection function. Hence how to provide an electric power protection circuit for a switch mode power supply is a big issue in the electronic industry.